sub lookup_subname {
  my($pkg, $sub) = @_;

  no strict 'refs';
  return ( grep { $pkg->can($_) eq $sub }
           keys %{"$pkg\::"} )[0];
}

sub foo;
sub _bar;

my %hash = ( foo => \&foo, bar => \&_foo );

for my $f (keys %hash) {
  my $name = lookup_subname __PACKAGE__, $hash{$f};
  printf "%s is %s\n",
         $f, ( $name =~ /^_/ ? 'private' : 'public' );
}
__output__

bar is private
foo is public